1. Field of the Invention
This invention relates to a nickel electrode which is incorporated as the positive electrode of a nickel/hydrogen secondary battery, nickel/zinc secondary battery, nickel/cadmium secondary battery, and the like, more specifically, to a nickel electrode for secondary battery, ensuring a provision of a battery which is increased in the packing density of an active material into a current collector, which realizes a high efficiency as an active material, and which has a high average discharge voltage.
2. Prior Art
Alkali secondary batteries, such as a nickel/hydrogen secondary battery, nickel/zinc secondary battery, nickel/cadmium secondary battery, and the like, have a structure in which a generating element and a specific alkali electrolytic solution are together sealed in a cell case. The generating element is formed by laminating a positive nickel electrode and a specific negative electrode via a separator. The nickel electrodes incorporated as a positive electrode are mainly classified into an sintered type and a non-sintered type.
Among these, a nickel electrode of the latter non-sintered type is generally manufactured by the following processes.
Specifically, Ni(OH).sub.2 powder which acts as an active material, an electroconductive material powder such as Ni powder, and an aqueous solution of a thickener such as an aqueous solution of carboxymethyl cellulose, are uniformly mixed each in a predetermined ratio, formulating, as required, a binder such as a PTFE dispersion, to prepare an active material-mixture paste. This mixture paste is filled into a current collector having a three dimensional network structure such as a foaming nickel plate so that the packing density accords with a designed value, which is then dried and rolled in order. The rolled product is cut into a specific size to produce a nickel electrode.
In this case, it is known that CoO powder or Co(OH).sub.2 powder is further added when the mixture paste is prepared. This is because of the following reason.
Specifically, illustrating a nickel electrode to which CoO powder or Co(OH).sub.2 powder are added, these Co compounds dissolve in an alkali electrolytic solution to form a complex ion. This complex ion precipitates as an electroconductive --CoOOH on Ni(OH).sub.2 powder as an active material and a current collector in the stage of charging for an initial activating treatment to form an electroconductive network, whereby the efficiency of Ni(OH).sub.2 as an active material is improved.
As CoO powder or Co(OH).sub.2 powder showing such effects, the following materials are conventionally used.
Specifically, as examples of Co(OH).sub.2 powder, dry powder of a precipitate prepared, for example, by a reaction of an aqueous solution of NaOH with an aqueous solution of CoSO.sub.4 is given. Also, CoO powder can be prepared by heat-treating the above Co(OH).sub.2 powder in a non-oxidizing atmosphere such as a nitrogen atmosphere.
The Co(OH).sub.2 powder has almost a hexagonal plate shape or undefined shape since the crystal structure of Co(OH).sub.2 belongs to a hexagonal system. The shape of the CoO powder derived from the aforesaid Co(OH).sub.2 powder is also similar to that of the Co(OH).sub.2 powder.
These powdery materials exist as a secondary aggregated substance of a primary particle having a particle diameter of 1 .mu.m or less. The secondary aggregated substance is served for the actual use when the above mixture paste is prepared.
As mentioned above, the CoO powder or Co(OH).sub.2 powder is dissolved once in an alkali electrolytic solution and precipitated as .beta.-CoOOH at the stage of charging of the initial activating treatment to form an electroconductive network on the surfaces of Ni(OH).sub.2 powder and of a current collector, and thereby to lower the polarization of the nickel electrode so that the CoO powder or Co(OH).sub.2 powder works to improve the efficiency of an active material.
Accordingly, it is desirable that the electroconductive network be not only formed on part of the surface of Ni(OH).sub.2 powder or the current collector, but also formed uniformly on the whole surfaces thereof so that the above effect can be sufficiently exhibited.
For realizing such a condition, it is preferable that the CoO powder or Co(OH).sub.2 powder be uniformly dispersed in the entire mixture paste without maldistribution.
However, Co(OH).sub.2 powder or CoO powder is a secondary aggregated substance formed of a micropowder having a hexagonal plate shape or undefined shape and a size of smaller than 1 .mu.m as mentioned above. When preparing the mixture paste, Co(OH).sub.2 powder or CoO powder conventionally used tends to be maldistributed in the paste in a state of a small nodule of a secondary aggregated substance when the mixture paste is prepared, exhibiting a problem that the uniform dispersion of the Co(OH).sub.2 powder or CoO powder is difficult.
Also, because the powder actually used in the step of preparing the mixture paste is a secondary aggregated substance, its tap density is small. Therefore, when producing a nickel electrode by filling a predetermined amount of a mixture paste prepared by formulating the powder in a predetermined amount, the relative amount of an active material to that of the mixture paste becomes small. The packing density of an active material in a current collector is small, which is not desirable for preparing a nickel electrode having high capacity.